The invention relates to a wire, preferably for bonding in microelectronics, comprising a copper core with a surface and a coating layer comprising aluminum superimposed over the surface of the copper core. The ratio of the thickness of the coating layer to the diameter of the copper core is in the range of from 0.05 to 0.2 and the ratio of the standard deviation of the diameter of the copper core to the diameter of the copper core is in the range of from 0.005 to 0.05. The ratio of the standard deviation of the thickness of the coating layer to the thickness of the coating layer is in the range of from 0.05 to 0.4 and the wire has a diameter in the range of from 100 μm to 600 μm. The invention further relates to a process for making a wire, to a wire obtainable by the process, to an electric device comprising at least two elements and at least the wire, to a propelled device comprising the electric device, and to a process of connecting two elements through the wire by wedge-bonding.
Bonding wires are used in the manufacture of semiconductor devices for electrically interconnecting an integrated circuit and a printed circuit board during semiconductor device fabrication. Further, bonding wires are used in power electronic applications to electrically connect transistors, diodes and the like with pads or pins of the housing. While bonding wires were made from gold in the beginning, nowadays less expensive materials, such as copper or aluminum, are used. While copper wire provides very good electric and thermal conductivity, wedge-bonding of copper wire has its challenges compared to wires made of aluminum. Moreover, copper wires are susceptible to oxidation of the wire.
With respect to wire geometry, most common are bonding wires of circular cross-section and bonding ribbons, which have a more or less rectangular cross-section. Both types of wire geometries have their advantages, making them useful for specific applications. Thus, both types of geometry have their share in the market. For example, bonding ribbons have a larger contact area for a given cross-sectional area. However, bending of the ribbons is limited and orientation of the ribbon must be observed when bonding in order to arrive at acceptable electrical contact between the ribbon and the element to which it is bonded. Turning to bonding wires, these are more flexible to bending. However, bonding involves either soldering or larger deformation of the wire in the bonding process, which may cause harm or even destroy the bonding pad and underlying electric structures of the element which is bonded thereto.
Some recent developments were directed to bonding wires having a core and a shell, which is, for example, a coating layer. As core material, copper or gold may be chosen because of high electric conductivity. With regard to the coating layer, aluminum is one of the more common choices. These core-shell bonding wires combine some of the advantages of the copper wire and some of the aluminum wire. Recent achievements make using standard wedge-bonding processes for such aluminum coated copper wires possible. Nevertheless, there is an ongoing need for further improving bonding wire technology with regard to the bonding wire itself and the bonding processes.